1. Field of the Invention
The invention relates to methods for refining molten metals and for recovering metals from slags. More particularly, the invention relates to electrochemical methods for refining molten metals and recovering metals from slags.
2. Description of the Prior Art
Requirements for metals having improved physical and mechanical properties necessitate production of metals having low, in the range of from about 100 ppm to less than about 5 ppm, concentrations of undesirable elements such as oxygen, hydrogen and sulfur.
Several technologies currently exist for removal of impurities such as oxygen, hydrogen and sulfur from molten metals. One of these molten metal refining techniques is vacuum degassing, a capital intensive technique limited by the vacuum pressure that can be created over the molten metal. Other known molten metal refining techniques are "gettering" techniques wherein a reactive metal forms more stable compounds with the impurity element than the original impurity compound present in the unrefined melt and, thereby, generates new compounds of the impurity element. Disadvantages of "gettering" techniques are the high cost of typical reactive metals, such as silicon, aluminum, manganese and calcium, as well as the presence of new impurity element compounds which can remain in the melt as inclusions if additional processing steps to remove the new impurity element compounds are not taken. Such additional processing steps introduce further complexity in the refining process and can produce undesirable effects such as incomplete removal of an impurity element and inclusion compounds and generate environmentally harmful by-products such as dust, slags and undesirable gases.
Existing electrochemical techniques for deoxidation of metals, such as that described by Iwase et al., "Electronically Driven Transport of Oxygen from Liquid Iron to CO+CO.sub.2 Gas Mixtures through Stabilized Zirconia", Metallurgical Transactions B, 12B, 517-524 (1981) have an oxygen removal flux which is rate limited by residual electronic transport in the electrolyte and are unacceptable from a commercial standpoint. No external electrodes or short-circuiting thereof is used. Under these conditions, residual electronic transport is one-tenth of oxygen ion transport. Other electrochemical deoxidation techniques, such as that described by Oberg et al., "Electrochemical Deoxidation of Induction-Stirred Copper Melts", Metallurgical Transactions B, 4, 75-82 (1973), require that an electric potential be applied between the melt and an electrode, as in an electrolysis process, to pump oxygen out of the melt and into the air and are, thus, less energy efficient than a galvanic cell operated without any external electromotive force.
The ever rising demand for metals coupled with an increasing scarcity of available mineral resources makes the salvaging of metal losses in metal oxide slags of great practical significance. A recent trend has been to increase the rate of metal production in pyrometallurgical operations by the use of pneumatic systems, such as basic oxygen steelmaking processes which have replaced earlier relatively quiescent slag-metal processes such as the open hearth steelmaking process. While pneumatic processes are desirable to increase productivity, they, in general, also increase loss of valuable metals and metal oxides, as is also well documented for non-ferrous metals such as tin and copper as described by Meyer et al., "Slag-Metal Emulsions and their Importance in BOF Steelmaking", Journal of Metals, 20, 35 (1968) and Floyd et al., "Flotation of metal during injection of gases into liquid slags", Transactions of the Institute of Mining and Metallurgy Sec. C, 82 C51 (1973).
Presently available methods for recovering metals from metal oxide slags include gaseous reduction of the slag as described in Subramanian et al., "Copper recovery by flotation", Journal of Metals, 24, 33 (1972); however, this technique is undesirable from the standpoint of contamination of the metal with the reducing gas and generation of environmentally harmful by-products such as dust and toxic gases.
Thus, there exists a need for inexpensive, energy-efficient, and environmentally sound molten metal refining techniques which do not generate impurity element compounds as a by-product which can be included in the refined metal. A need also exists for energy-efficient and environmentally sound techniques for metal recovery from slags.